Single-sided circuit board and method for manufacturing the same

ABSTRACT

Holes ( 40   a ) are formed with a laser beam through an insulating substrate ( 40 ) on which a metallic layer ( 42 ) is formed and via holes ( 36   a ) are formed by filling up the holes ( 40   a ) with a metal ( 46 ). After the via holes ( 36   a ) are formed, a conductor circuit ( 32   a ) is formed by etching the metallic layer ( 42 ) and a single-sided circuit board ( 30 A) is formed by forming projecting conductors ( 38   a ) on the surfaces of the via holes ( 36   a ). The projecting conductors ( 38   a ) on the circuit board ( 30 A) are put on the conductor circuit ( 32   b ) of another single-sided circuit board ( 30 B) with adhesive layers ( 50 ) composed of an uncured resin in-between and heated and pressed against the circuit ( 32   b ). The projecting conductors ( 38   a ) get in the uncured resin by pushing aside the resin and are electrically connected to the circuit ( 32   b ). Since single-sided circuit boards ( 30 A,  30 B,  30 C, and  30 D) can be inspected for defective parts before the boards ( 30 A,  30 B,  30 C, and  30 D) are laminated upon one another, only defectless single-sided circuit can be used in the step of lamination.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a single-sided circuit board and amanufacturing method therefor, and more particularly to a single-sidedcircuit board used for manufacture of a multilayer printed circuit boardhaving an interstitial via hole (IVH) structure and a manufacturingmethod therefor.

2. Description of the Related Art

A conventional multilayer printed circuit board is constituted by alaminate obtained by alternately laminating copper-clad laminates andprepregs. The laminate has a surface on which a surface wiring patternhas been formed. Moreover, an internal wiring pattern is formed betweeninterlayer insulating layers. Through holes are, by punching formed inthe direction of the thickness of the laminate to establish the electricconnections among internal wiring patterns or among internal wiringpatterns and the surface wiring patterns.

The multilayer printed circuit board having the above-mentioned throughhole structure must have regions for forming the through holes.Therefore, the density at which elements are mounted cannot be easilyraised. As a result, there arises a problem in that the multilayerprinted circuit board having the through hole structure cannot easilyaddress needs for considerable reductions in the sizes of the portableelectronic apparatuses, realizing narrow pitch packages and practicaluse of MCM.

As an alternative to the foregoing multilayer printed circuit boardhaving the through hole structure, a multilayer printed circuit boardhas recently received attention which is formed into a full-thicknessinterstitial via-hole (IVH) structure which is able to easily addressraising of the density.

The multilayer printed circuit board having the full-thickness IVHstructure is a printed circuit board having a structure in which viaholes for electrically connecting conductive layers to one another areformed in each of the interlayer insulating layers which constitute thelaminate. That is, the foregoing printed circuit board has the via holes(buried via holes or blind via holes) which do not penetrate thesubstrate on which the circuit is formed and which electrically connectthe internal wiring patterns to one another or the internal wiringpatterns and the surface wiring patterns to one another. Therefore, themultilayer printed circuit board having the IVH structure is free from anecessity of specially forming regions for forming the through holes.Therefore, arbitrary layers can freely be connected to one anotherthrough small via holes. As a result, size reduction, high density andhigh-speed propagation of signals can be easily realized.

The multilayer printed circuit board having the IVH structure ismanufactured by a process arranged, for example, as shown in FIG. 6.

Initially, a material having a structure in which an aramide nonwovenfabric cloth is impregnated with epoxy resin is employed as prepreg 112.Then, an operation for forming holes in the prepreg 112 is performed byusing carbon dioxide gas laser. Then, conductive paste 114 is enclosedin obtained hole portions 112 a (see FIG. 6(A)).

Then, copper foil 116 is laminated on each of the two sides of theprepreg 112, and then heat and pressure are applied to the prepreg 112having the copper foil 116 by heat pressing. Hence it follows that theepoxy resin and the conductive paste of the prepreg 112 are hardened sothat the electrical connection between the two copper foil members 116on the two sides of the prepreg 112 is established (see FIG. 6(B)).

Then, the copper foil 116 on each side is patterned by an etching methodso that a hard and double-sided substrate having via holes is obtained(see FIG. 6(C)).

Then, the obtained double-sided substrates are used as core layers toform a multilayer structure. Specifically, the prepreg and copper foilare sequentially laminated on the two sides of the foregoing core layerwhile the prepreg and the copper foil are being aligned. Then, heatpressing is again performed, and then the uppermost copper foil 116 isetched. Thus, a four-layer substrate is obtained (see FIGS. 6(D) and6(E)). When a structure having a large number of layers is formed, theforegoing process is repeated. Thus, a six-layer substrate or aneight-layer substrate can be obtained.

The foregoing conventional technique, however, must repeat thelaminating process using heat pressing and the process for patterningthe copper foil by performing the etching operation. Therefore, themanufacturing process becomes too complicated and considerable time isrequired to complete the manufacturing operation.

If the multilayer printed circuit board having the IVH structure whichcan be obtained by the above-mentioned manufacturing method encountersonly one defective portion (only one defective process) in thepatterning operation during the manufacturing process, the overallcircuit board, which is the final product, becomes a defective productHence it follows that the manufacturing yield deteriorates excessively.

To overcome the above-mentioned problems, an object of the presentinvention is to provide a high-density multilayer printed circuit boardhaving the IVH structure which can be efficiently manufactured and whichpermits a satisfactory high manufacturing yield.

DISCLOSURE OF THE INVENTION

To achieve the above-mentioned object, a single-sided circuit board inthe present invention is structured as follows. A single-sided circuitboard according to the present invention is characterized in that aninsulating base member on either side of which a conductive circuit isformed, wherein said insulating base member has non-penetrating holeswhich reach said conductive circuit, via holes are formed by enclosingelectrolytic plating in said non-penetrating holes, projectingconductors made of conductive paste or metal having a low melting pointare formed on the surfaces of said via holes formed on the surface ofsaid insulating base member opposite to the surface on which saidconductive circuit has been formed.

In accordance with the more preferred teaching of the present invention,said non-penetrating holes are characterized by being completely filledwith electrolytic plating.

In accordance with the more preferred teaching of the present invention,said non-penetrating holes are characterized by being filled with theelectrolytic plating to an average depth of 50% or higher of the depthof said non-penetrating holes and lower than 100% of the same.

In accordance with the more preferred teaching of the present invention,said insulating base member is characterized as an organic insulatingmember.

In accordance with the more preferred teaching of the present invention,said electrolytic plating is characterized as copper electrolyticplating.

In accordance with the more preferred teaching of the present invention,said insulating member is characterized as a single-sided copper-cladlaminate.

A method of manufacturing a single-sided circuit board according to thepresent invention is characterized by at least comprising the steps (1)to (4):

(1) forming non-penetrating holes which reach a metal layer by a laserprocess in a insulating base member having said metal layer formed oneither side thereof;

(2) enclosing electrolytic plating into said non-penetrating holesformed in step (1) to form via holes;

(3) forming a conductive circuit by etching said metal layer; and

(4) forming projecting conductors made of conductive paste or metalhaving a low melting point on the surfaces of said via holes formed onthe surface of said insulating base member to the surface on which saidconductive circuit has been formed so that a single-sided circuit boardis obtained.

A method of manufacturing a single-sided circuit board according to thepresent invention is characterized by at least comprising the steps (1)to (3):

(1) forming non-penetrating holes which reach a metal layer by a laserprocess in a insulating base member having said metal layer formed oneither side thereof;

(2) enclosing electrolytic plating into said non-penetrating holesformed in step (1) to form via holes and forming projecting conductorsmade of conductive paste or metal having a low melting point on thesurfaces of said via holes formed on the surface of said insulating basemember opposite to the surface on which said conductive circuit has beenformed; and

(3) etching metal layer to form a conductive circuit so that asingle-sided circuit board is obtained.

A method of manufacturing a single-sided circuit board in accordancewith the more preferred teaching of the present invention ischaracterized in that said step (1) is performed such that a film isbonded to insulating base member having the metal layer formed on eitherside thereof and non-penetrating holes which penetrate said film andsaid insulating base member to reach said metal layer are formed by alaser process.

A method of manufacturing a single-sided circuit board in accordancewith the more preferred teaching of the present invention ischaracterized in that said insulating base member is an organicinsulating base member.

A method of manufacturing a single-sided circuit board in accordancewith the more preferred teaching of the present invention ischaracterized in that said insulating base member is a single-sidedcopper cladding laminate.

A method of manufacturing a single-sided circuit board in accordancewith the more preferred teaching of the present invention ischaracterized in that said electrolytic plating is copper electrolyticplating.

A method of manufacturing a single-sided circuit board in accordancewith the more preferred teaching of the present invention ischaracterized in that said projecting conductors are formed by printingconductive paste.

A method of manufacturing a single-sided circuit board in accordancewith the more preferred teaching of the present invention ischaracterized in that said projecting conductors are formed by printingmetal paste having a low melting point, plating of metal having a lowmelting point or immersion of metal having a low melting point intomolten solution.

The single-sided circuit board according to the present invention isarranged such that the single-sided circuit boards each having aconductive circuit which incorporates a predetermined wiring patternformed thereon are previously and individually manufactured.

These single-sided circuit boards are laminated to other substratesthrough an adhesive layer and are integrated by the heat pressing.Therefore, inspection for detecting whether or not the conductivecircuit or the like has a defective portion can be performed before thesingle-sided circuit boards are laminated. Hence it follows that onlysingle-sided circuit boards free from any defect can be used in thelaminating process. That is, the manufacturing method is able to reducedefects in the manufacturing process. As a result, the multilayerprinted circuit board having the IVH structure can be manufactured witha high manufacturing yield.

The method of manufacturing a multilayer printed circuit board havingthe IVH structure used the single-sided circuit board according to thepresent invention is not required to repeat the heating press operationwhile the prepreg is being laminated as distinct from the conventionaltechnique. That is, the present invention enables the heat pressingoperation to be completed at a time such that a plurality of thesingle-sided circuit boards are laminated through the adhesive agentsplaced on the single-sided circuit boards. Therefore, the manufacturingmethod according to the present invention is free from a necessity forrepeating the laminating process, in which the complicated heat press isperformed, and the patterning process. Hence it follows that themultilayer printed circuit board having the IVH structure canefficiently be manufactured.

In the present invention, formation of the non-penetrating holes in theorganic insulating substrate is performed by laser machining. Thestructure of the present invention is able to eliminate the necessity offorming holes in the organic adhesive agent, an organic insulatingsubstrate and an adhesive agent by performing a laser process. That is,after holes have been formed in the organic insulating substrate byperforming the laser process, the organic adhesive layer can be formedon the single-sided circuit substrate or the substrate having theconductor circuit.

That is, the single-sided circuit board according to the presentinvention is structured such that the projecting conductors insertedinto the organic adhesive layer during the heat pressing establish theconnection of the conductor circuits. Therefore, previous formation ofthe conducting hole in the organic adhesive layer is not required. Theorganic adhesive layer may be formed at the final heat pressing process.As a result, the desmear process, which is performed after the hole hasbeen formed, may be performed before the formation of the organicadhesive layer. Hence it follows that the desmear process does not erodethe adhesive layer.

Also, the adhesive layer can be formed on the single-sided circuit boardor the substrate having the conductive circuit after the non-penetratinghole has been formed in the insulating base member by performing thelaser process and the non-penetrating hole has been filled with theeectrolytic plating. Therefore, the electrolytic plating solution andthe adhesive layer are not made contact with each other. As a result,the erosion and contamination of the adhesive layer with the platingsolution can be prevented.

Since the adhesive layer is not hardened until the heat pressing processwhich is the final process is performed, the organic adhesive layereasily deteriorates owing to the desmear process and the platingsolution. The present invention is characterized in that the foregoingproblem can be prevented and a reliable substrate can be easily formed.

Furthermore, projecting conductors made of conductive paste or metalhaving a low melting point are formed on the surfaces of via holesformed on the surface of said insulating base member opposite to thesurface on which conductive circuit has been formed. Therefore, sinceconductive paste or metal having a low melting point is deformed duringthe heat pressing and uneven height of the electric plating can beabsorbed, the multilayer printed circuit board, whereby the defectiveconnection can be prevented which is highly reliable in connection, isobtained.

Moreover, according to the single-sided circuit board of the presentinvention, a necessity for previously forming a conducting hole in theadhesive layer can be eliminated. Therefore, defective conduction causedfrom deviation in the positions of the hole in the adhesive layer andthe projecting conductors provided for the organic insulating basemember can be prevented.

In the single-sided circuit board according to the present invention,the projecting conductors are formed on the via holes filled with theelectrolytic plating. Therefore, electrical connection between the upperand lower conductive layers in the multilayer printed circuit substratecan be easily established by penetrating the relatively thin organicadhesive layer. Therefore, the height and the diameter of eachprojecting conductor can be reduced. Hence it follows that the pitchbetween adjacent projecting conductors can be shortened. Therefore, thepitch between adjacent via holes can also be shortened. As a result,addressing to the raising of the density can be permitted. Since the viaholes are filled with the electrolytic plating, the resistance valuebetween the upper and the lower conductor layers can be lowered.

Techniques for connecting the upper and lower conductors to each otherby penetrating the resin insulating layer of the multilayer printedcircuit board have been disclosed in Japanese Patent Laid-Open No.7-14628, Japanese Patent Laid-Open No. 7-106756, Japanese PatentLaid-Open No. 7-231167, Japanese Patent Laid-Open No. 8-172270 andJapanese Patent Laid-Open No. 8-288649. The foregoing techniques aredifferent from the technique with which the projecting conductors areformed on the via holes filled with the electrolytic plating. Therefore,the effect of the present invention cannot be obtained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a vertical cross sectional view showing a multilayer printedcircuit board according to an embodiment of the present invention;

FIG. 2 is a diagram showing a process for manufacturing a core substratewhich constitutes the multilayer printed circuit board according to theembodiment of the present invention;

FIG. 3 is a diagram showing a process for manufacturing a single-sidedcircuit board which constitutes the multilayer printed circuit boardaccording to the embodiment of the present invention;

FIG. 4 is a diagram showing a process for manufacturing the single-sidedcircuit board which constitutes the multilayer printed circuit boardaccording to the embodiment of the present invention;

FIG. 5 is a diagram showing a process for manufacturing the multilayerprinted circuit board according to the embodiment of the presentinvention;

FIG. 6 is a diagram showing a process for manufacturing a conventionalmultilayer printed circuit board;

FIG. 7 is a diagram showing a process for manufacturing the coresubstrate which constitutes the multilayer printed circuit boardaccording to the embodiment of the present invention;

FIG. 8 is a diagram showing a process for manufacturing the coresubstrate which constitutes the multilayer printed circuit boardaccording to the embodiment of the present invention; and

FIG. 9 is an enlarged photograph showing a metal structure of the crosssection of a via hole in the multilayer printed circuit board accordingto the embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

A single-sided circuit board and a multilayer printed circuit board anda manufacturing method therefor used single-sided circuit boardaccording to an embodiment of the present invention will now bedescribed with reference to the drawings.

FIG. 1 shows a vertical cross section of a multilayer printed circuitboard having a full-thickness IVH structure. A multilayer printedcircuit board 10 is a multilayer printed circuit board incorporating acore substrate 20 disposed in a central portion of the multilayerprinted circuit board 10 and single-sided circuit boards 30A, 30B, 30Cand 30D of the present invention, two substrates of which are formed onthe upper surface of the core substrate 20 and the lower surface of thesame, respectively.

Conductive circuits 32 a, 32 b, 32 c and 32 d each having apredetermined pattern are formed on the surfaces of the correspondingsingle-sided circuit boards 30A, 30B, 30C and 30D. Adhesive layers 34are formed on other surfaces of the same. The core substrate 20 and thesingle-sided circuit boards 30A, 30B, 30C and 30D are bonded to oneanother through the adhesive layers 34. The single-sided circuit boards30A, 30B, 30C and 30D have corresponding via holes 36 a, 36 b, 36 c and36 d formed by enclosing electrolytic copper plating. Projectingconductors (hereinafter called “bumps”) 38 a, 38 b, 38 c and 38 d madeof metal, such as solder or an indium alloy or conductive paste, areformed on the foregoing via holes (the surfaces of the via holesopposite to the surfaces on which the conductive circuits are formed).

That is, in the multilayer printed circuit board 10, a conductivecircuit 32 a of the lowermost single-sided circuit board 30A isconnected to a bump 38 a through the via hole 36 a. The bump 38 a ismade to contact with a conductive circuit 32 b of the single-sidedcircuit board 30B to establish the connection between the two circuits.The bump 38 b connected to the conductive circuit 32 b through the viahole 36 b is made to contact with the through hole 24 (a via hole ismade after multilayer structure is formed) of the core substrate 20 sothat conduction is realized. The through hole 24 of the core substrate20 is connected to the bump 38 c of the upper single-sided circuit board30C. The conductive circuit 32 c connected to the bump 38 c through thevia hole 36 c is connected to the bump 38d of the uppermost single-sidedcircuit board 30D. The bump 38 d is connected to the conductive circuit32 d through the via hole 36 d. An electronic element, such as a bearchip, can be mounted on either surface or two surfaces of the uppermostsingle-sided circuit board 30D. Thus, the conductive circuit 32 a of thelowermost single-sided circuit board 30A of the multilayer printedcircuit board and the chip element (not shown) mounted on the conductivecircuit 32 of the uppermost single-sided circuit board 30D are connectedto each other through the via holes 36 a, 36 b, 36 c and 36 d. Theforegoing via holes constitute interstitial via holes.

The method of manufacturing the multilayer printed circuit board 10using single-sided circuit board according to the present invention willnow be continued. A method of manufacturing the core substrate 20 willnow be described with reference to FIG. 2.

The core substrate may be a known rigid substrate, such as an epoxyresin substrate having a glass cloth base member or a BT(Bismaleimide-Triazine) resin substrate having a glass cloth basemember.

Specifically, in process (A) shown in FIG. 2, a starting material iscopper-clad laminates incorporating a substrate 22 which is made of BT(Bismaleimide-Triazine) resin and which has two sides on each of whichcopper foil 21 is bonded. In process (B), holes 22 a serving as throughholes are formed in the substrate 22 by punching, and then electrolessplating is performed so that the inner surfaces of the holes 22 a areapplied with copper plating. Thus, through holes 24 are formed.

In process (C), etching resist (not shown) is previously applied, andthen an etching process is performed to remove unnecessary portions ofthe copper foil 21. As a result, a predetermined conductive circuit 25is formed.

In process (D), the surfaces of the conductive circuit 25 and thethrough holes 24 are subjected to blacking-reducing process so as to becoarsened.

In process (E), resin 26 which must be enclosed is uniformly applied byusing a roll coater, and then the resin which must be enclosed ishardened. Then, the resin which must be enclosed is ground with a beltsander or the like until the conductive circuit 25 is exposed on thesurface. As a result, a core substrate 20 having two flat surfaces ismanufactured.

The core substrate 20 is brought to a state in which the inside portionof the through holes 24 and two sides 25 a of the conductive circuit 25are coarsened. As a result, the adhesiveness between the conductivecircuit 25 and the resin 26 which must be enclosed can be improved.Hence it follows that occurrence of a crack can be prevented whichoccurs in the adhesive layer 34 described with reference to FIG. 1 andwhich starts at the interface between the conductive circuit 25 and theresin 26 which must be enclosed.

Description of the method of manufacturing the single-sided circuitboard 30 according to the present invention will now be continued withreference to FIGS. 3 and 4. In process (A) shown in FIG. 3, aninsulating base member 40 having a metal layer 42 formed on eithersurface thereof is used as a starting material. The insulating basemember 40 must be an organic insulating base member. Specifically, it ispreferable that a base member is selected from rigid laminates includingan aramide nonwoven fabric cloth-epoxy resin base member, a glass clothepoxy resin base member, an aramide nonwoven fabric cloth-polyimide basemember and a bismaleimide-triazine resin base member or films includinga polyphenylene (PPE) film and a polyimide (PI) film.

It is preferable that the insulating base member 40 is a rigid laminatedbase member. In particular, it is preferable that a single-sided circuitboard is employed. The reason for this lies in that displacement of thepositions of the wiring pattern and the via holes can be preventedduring handling which is performed after the metal layer 42 has beenetched. That is, excellent accuracy of the position can be realized.

The metal layer 42 formed on the insulating base member 40 may be copperfoil. The copper foil may be subjected to a matting process in order toimprove the adhesiveness. The single-sided circuit board is a substratewhich can be obtained by heat pressing a laminate of prepreg and copperfoil, the prepreg being in the form of a B-stage constituted by causinga glass cloth to be impregnated with thermosetting resin, such as epoxyresin, phenol resin or bismaleimide-triazine resin. The single-sidedcircuit board is a rigid substrate which can easily be handled and whichis an advantage substrate from the viewpoint of cost reduction. Metalmay be evaporated on the surface of the insulating base member 40,followed by forming a metal layer by performing electrolytic plating.

The thickness of the insulating base member 40 is 10 μm to 200 μm,preferably 15 μm to 100 μm. The optimum thickness is 20 μm to 80 μm tomaintain insulating characteristics. If the thickness is smaller thanthe above-mentioned ranges, the strength is decreased excessively toperform easy handling. If the thickness is too large, formation of smallvia holes is inhibited and enclosing of the conductive material cannoteasily be performed.

The thickness of the metal layer 42 is 5 μm to 35 μm, preferably 8 μm to30 μm, and more preferably 12 μm to 25 mm. If the thickness is toosmall, a hole attempted to be formed by a laser process as describedlater is undesirably formed into a through hole. If the thickness is toolarge, a fine pattern cannot be formed by the etching process.

Then, the laser irradiation process is performed to form non-penetratingholes 40 a in the insulating base member 40 (process (B)). The laserprocessing machine may be a carbon dioxide gas laser processing machine,an UV laser processing machine or an excimer laser processing machine.It is preferable that the caliber of the laser processing machine is 20μm to μm. The carbon dioxide gas laser processing machine exhibits highprocessing speed, enabling the cost of the process to be reduced.Therefore, the foregoing laser processing machine is a most suitablemachine from a viewpoint of industrial use. Therefore, the foregoinglaser processing machine is a most suitable machine for the presentinvention. When the carbon dioxide gas laser processing machine isemployed, resin existing in the non-penetrating hole 40 a and slightlymelted onto the surface of the metal layer 42 is easily left. Therefore,it is preferable that a desmear process is performed to maintain thereliability of the connection.

Then, an electrolytic plating is enclosed in the non-penetrating holes40 a formed by the laser process so that via holes 36 a are formed(process (E)). As an alternative to this, conductive paste may beenclosed or a portion of electrolytic plating may be enclosed, followedby enclosing conductive paste into a residual portion. The electrolyticplating may be, for example, copper, gold, nickel or solder plating.Most suitable electrolytic plating is copper electrolytic plating. Inthis case, the formation of the bumps can be performed simultaneouslywith the process.

The conductive paste may be conductive paste made of metal particlesmade of one or more materials selected from silver, copper, gold, nickeland solder. As the metal particles, a material may be employed which isobtained by coating the surfaces of the metal particles with differentmetal. Specifically, metal particles may be employed which are obtainedby coating the surfaces of copper particles with noble metal selectedfrom gold and silver.

It is preferable that the conductive paste is organic conductive pasteobtained by adding thermosetting resin, such as epoxy resin, phenolresin or polyphenylene sulfide (PPS), to the metal particles.

In this embodiment, the small holes each having a diameter of 20 μm to150 μm are formed by the laser process and enclose the electrolyticplating, which is an advantage since air bubbles easily release comparedwith the enclosure of the conductive paste.

The electrolytic plating is performed as the lead for the metal layer 42formed on the insulating base member 40. Since the metal layer 42 isformed on the overall surface of the insulating base member 40, thedensity of the electric field can be uniform. In this embodiment, thenon-penetrating holes are completely enclosed by electrolytic plating.Therefore, the non-penetrating holes can be enclosed with theelectrolytic plating such that a uniform height is realized. It ispreferable that the surface of the metal layer 42 in each of thenon-penetrating holes 40 a is subjected to an activating process usingacid or the like. When plating is performed, it is preferable thatdeposition of the electrolytic plating on the surface of the metal layer42 provided for the organic insulating base member 40 is prevented byapplying a mask 48 on the metal layer 42 in process (C). As analternative to this, in process (D), two insulating base members 40 arelaminated and brought into hermetic contact with each other to preventcontact with the plating solution during the electrolytic platingprocess.

After the electrolytic plating has been completed, a portion of theelectrolytic plating (metal 46) projecting over the non-penetratingholes 40 a may be removed by grinding or the like to flatten the surfacein process (F) shown in FIG. 4. The grinding process may be performed byusing a belt sander or by buffing.

In process (G), a process which is performed before the metal layer 42is etched to form the conductive circuit and which is a process foreasily forming a fine pattern is carried out with which thenon-penetrating holes are previously formed by a laser process, followedby etching the overall surface of the metal layer 42 to reduce thethickness to about 1 μm to about 10 μm, preferably about 2 μm to about 8μm.

As shown in process (H), a mask having a predetermined pattern isapplied, and then the metal layer 42 is etched to form the conductivecircuit 32 a. Initially, a photosensitive dry film is applied orliquid-type photosensitive resist coat is applied. Then, exposure anddevelopment are performed to conform to the predetermined circuitpattern so that an etching resist is formed. Then, the metal layer inthe portions in which the etching resist is not formed is etched so thata conductor pattern is formed. It is preferable that the etching isperformed by using solution of at least one type of material selectedfrom sulfuric acid-hydrogen peroxide, persulfate, cupric chloride andferric chloride.

Note that the uppermost pattern may be formed by etching the metal layerafter the laminate pressing process has been completed. When the metallayer is etched after the laminate pressing process has been completed,an advantage can be realized in that heat pressing can be performed withuniform pressure because the surfaces which must be pressed are flatsurfaces.

It is preferable that the surface of the conductive circuit 32 a issubjected to a coarsening process. The reason for this lies in that theadhesiveness with the adhesive layer 34 described with reference to FIG.1 can be improved in the foregoing case to prevent separation(delamination). The coarsening process may be, for example, a softetching process, a blackening (oxidizing) reducing process, formation ofneedle-shape alloy plating (“INTERPLATE” which is trade name of Ebara)of copper-nickel-phosphorus or surface coarsening process using etchingsolution “MECH ETCHBOND” which is trade name of Mech.

In process (I), bumps 38 a are formed on the surfaces of the via holes36 a opposite to the surface on which the conductive circuit 32 a hasbeen formed. The bumps 38 a can be formed by, for example, a method withwhich conductive paste is screen-printed by using a metal mask havingopenings formed at predetermined positions, a method with which paste ofsolder which is metal having a low melting point is printed, a methodwith which solder plating is performed or a method with which immersionin solution in which solder has been melted is performed.

The metal having the low melting point may be Pb—Sn type solder, Ag—Sntype solder or indium solder.

It is preferable that the height of each bump from the insulating 30base member 40 is 3 μm to 60 μm. If the height is smaller than 3 μm,absorption of dispersion of the heights of the bumps by virtue ofdeformation of the bumps cannot be realized. If the height is largerthan 60 μm, the resistance value is raised excessively. To a worseextent, the bumps are expanded in the lateral direction when the bumps35 have been deformed, causing short circuit to occur.

In the foregoing state or before the bumps are formed, inspections ofthe conductive circuit 32 a and the via holes 36 a can be performed. Theconventional multilayer printed circuit board permits the inspection ofthe conductive circuit to be performed only after lamination has beenperformed, that is, after completion. As compared with this, whether ornot the single-sided circuit board 30A has a defect can be inspectedbefore the laminating operation. Since only single-sided circuit board30A free from any defect can be used in a laminating process to bedescribed later, a satisfactory high yield of the multilayer printedcircuit board can be obtained.

The single-sided circuit board of the present invention laminates pluralsingle-sided circuit boards to each other or is laminated to the coresubstrate 20 shown in FIG. 2. The adhesive in this case, for example,shown in the process (J), coats the overall surface of the insulatingbase member 40 adjacent to the bumps 38 a with resin. Then, the resin isdried so that an adhesive layer 34 constituted by non-hardened resin isformed.

The adhesive layer 34 may be formed by coating the overall surface ofthe single-sided circuit board on which the conductive circuit has beenformed, the overall opposite surface or the overall surface of substrate20 on which the conductive circuit 25 b has been formed, the substrate20 being a substrate having a conductive circuit. Therefore, a necessityfor forming holes for establishing conduction in the adhesive layer canbe eliminated.

It is preferable that the adhesive layer 34 is made of organic adhesiveagent. It is preferable that the organic adhesive agent is a resinselected from epoxy resin, polyimide resin, thermosetting polyphenyleneether (PPE), composite resin of the epoxy resin and the thermosettingresin, composite resin of epoxy resin and silicon resin or BT resin.

The non-hardened resin which is the organic adhesive agent can beapplied by using a curtain coater, a spin coater, a roll coater or aspray coater or by screen printing. Also the formation of the adhesivelayer can be performed by laminating an adhesive sheet. It is preferablethat the thickness of the adhesive layer is 5 μm to 50 μm. To facilitatehandling, it is preferable that the adhesive layer is pre-cured.

The process of laminating the core substrate 20 described with referenceto FIG. 2 and the single-sided circuit board 30 described with referenceto FIGS. 3 and 4 will continuously be described with reference to FIG.5.

In process (K), the single-sided circuit board 30A, the single-sidedcircuit board 30B, 30C and 30D formed by a process similar to theabove-mentioned process and the core substrate 20 are laminated. All ofthe single-sided circuit boards 30A, 30B, 30C and 30D and the coresubstrate 20 must be substrates subjected to inspection for a defectiveportion. Initially, the single-sided circuit board 30B is placed on theorganic adhesive layer 34 of the single-sided circuit board 30A, whilethe core substrate 20 is placed on the organic adhesive layer 34 of thesingle-sided circuit board 30B. The foregoing substrates are laminatedon the core substrate in such a manner that the single-sided circuitboards 30C and 30D are inverted, that is, the organic adhesive layer 34of the single-sided circuit board 30C faces the core substrate 20.Moreover, the organic adhesive layer 34 of the single-sided circuitboard 30D faces the single-sided circuit board 30C. The lamination isperformed while position alignment is being performed by inserting aguide pin (not shown) into a guide hole (not shown) formed around thesingle-sided circuit board 30 and the core substrate 20. A portion ofcircle C of the laminated substrate shown in the drawing is indicated byenlarging the same in (M). The position alignment may be performed by animage process.

Finally, in process (L), the laminated substrates are heated to 150° C.to 200° C. by using a heating press and applied with pressure of 5kgf/cm² to 100 kgf/cm², preferably 20 kgf/cm² to 50 kgf/cm². Thus, thesingle-sided circuit boards 30A, 30B, 30C and 30D and the core substrate20 are integrated into a multilayer structure by one press moldingoperation. A portion of circle C of the laminated substrates shown inthe drawing is shown in (N). Since the pressure is applied, the bump 38a of the single-sided circuit board 30A squeezes out the non-hardenedresin (the insulating resin) existing between the bump 38 a and theconductive circuit 32 b adjacent to the single-sided circuit board 30B.Thus, the bump 38 a is brought into contact with the conductive circuit32 b so that the connection between the bump 38 a and the conductivecircuit 32 b is established. Similarly, the bumps 38 b, 38 c and 38 d ofthe single-sided circuit board 30B, 30C and 30D and the conductivecircuit are connected to one another. Since also heat is appliedsimultaneously with exertion of the pressure, the adhesive layer 34 ofthe single-sided circuit board 30A is hardened so that strong adhesionis realized with the single-sided circuit board 30B. It is preferablethat the heating press is vacuum heating press. As a result, themultilayer printed circuit board 10 described with reference to FIG. 1can be manufactured.

Another embodiment will now be described with reference to FIG. 7.

In process (B), a protective film 100 which is usually used as a maskserving in a process for printing conductive paste is bonded to asingle-sided copper clad laminate 40 which has been prepared in process(A) shown in FIG. 7. In process (C), an operation for laser-processingthe single-sided copper-clad laminate 40 is performed so thatnon-penetrating holes are formed. For the protective film 100, forexample, a polyethylene terephthalate film (PET) having an adhesivelayer formed on the surface thereof may be employed. Deposition ofplating onto the metal layer 42 must be prevented by bonding a mask 48in process (D). As an alternative to this, the metal layers 42 arebrought into hermetic contact with each other in process (E) to preventcontact with the electrolytic plating solution. In process (F) a portionof the non-penetrating holes is filled with electrolytic plating 46. Inprocess (G) conductive paste 460 is enclosed in residual spaces. In theforegoing embodiment, dispersion of the heights of the electrolyticplating can be corrected by using conductive paste 460 so that theheights of the bumps are uniform. As an alternative to the foregoingconductive paste, the metal having the low melting point can beenclosed.

An average charging rate of the electrolytic plating in thenon-penetrating holes (heights t of electrolytic plating×100/depth T ofthe non-penetrating holes: refer to FIG. 8(L).) is not lower than 50%nor lower than 100%, preferably 55% to 95%, more preferably 60% to 90%.

The conductive paste 460 enclosed in the opening portions of theprotective film 100 is formed into bumps. In process (H) a film 101 forprotecting the conductive paste 460 is bonded. Then, the metal layer 42is etched so that a conductive circuit is formed. Then, in the process(I) the film 101 is removed to expose the bumps formed by the conductivepaste 460 so that a single-sided circuit board 30E of the presentinvention is obtained.

It is preferable that the bumps made of the conductive paste is in asemi-hardened state. Since the conductive paste is hard if it is in thesemi-hardened state, the conductive paste is able to penetrate theorganic adhesive layer softened during the heat pressing process.

When the pressing process is performed, the conductive paste isdeformed, causing the area of contact to be enlarged. As a result, theconduction resistance can be lowered and dispersion of the heights ofthe bumps can be corrected. FIG. 9 shows an enlarged photograph of thestructures of the via hole and the bump portion.

Then, the single-sided circuit substrate 30E obtained by performingsteps (A) to (I) are laminated in process (J) so that three layers foreach side are placed opposite to each other through the adhesive layers80. The lamination is performed while position alignment is beingperformed by inserting a guide pin (not shown) into a guide hole (notshown) formed in the peripheries of the single-sided circuit board 30and the core substrate 20. The position alignment may be performed byperforming an image process.

Then, the multilayer printed circuit board 10 structured in process (K)may be manufactured by performing heat pressing.

In the foregoing embodiment, the multilayer printed circuit boards havefour laminated single-sided circuit boards 30 and six laminatedsingle-sided circuit boards 30, respectively. The single-sided circuitboard according to the present invention may be applied to a multilayerprinted circuit board having three layers or five or more layers. Thesingle-sided circuit boards according to the present invention may belaminated on a single-sided printed circuit board, a double-sidedprinted circuit board, a double-sided through-hole printed circuit boardor amultilayer printed circuit board to manufacture a multilayer printedcircuit board.

Although the foregoing embodiment has the structure that the holes forforming the via holes are formed by the laser process, the holes may beformed by a mechanical method, such as drilling, punching or the like.

The multilayer printed circuit board manufactured by the single-sidedcircuit board according to the present invention may be subjected to avariety of usual processes to which the printed circuit board has beensubjected. For example, formation of solder resist on the surface,nickel/gold plating, soldering, hole formation, a cavity forming processand plating of through holes may be performed.

As described above, the multilayer printed circuit board and amanufacturing method therefor according to the present invention arearranged such that the single-sided circuit boards each having aconductive circuit which incorporates a predetermined wiring patternformed thereon are previously and individually manufactured. Therefore,inspection for detecting whether or not the conductive circuit or thelike has a defective portion can be performed before the single-sidedcircuit boards are laminated. Hence it follows that only single-sidedcircuit boards free from any defect can be used in the laminatingprocess. That is, the manufacturing method according to the presentinvention is able to reduce defects in the manufacturing process. As aresult, the multilayer printed circuit board having the IVH structurecan be manufactured with a high manufacturing yield by the single-sidedcircuit board according to the present invention.

Also, it is not required by single-sided circuit board according to thepresent invention to repeat the heating press operation while theprepreg is being laminated as distinct from the conventional technique.That is, the present invention enables heat pressing operation to becompleted at a time such that a plurality of the single-sided circuitboards are laminated through the adhesive agents placed on thesingle-sided circuit boards. Therefore, it is free from a necessity forrepeating the laminating process in which the complicated heat press isperformed and the patterning process. Hence it follows that themultilayer printed circuit board having the IVH structure canefficiently be manufactured. Since the single-sided circuit boards areintegrated with physical force exerted at the time of the pressingoperation, the reliability of the connection can be improved.

Furthermore, in the present invention, in order to manufacture thesingle-sided circuit board, formation of the non-penetrating hole in theorganic insulating base member is performed by the laser process. Theprevious formation of the conducting hole in the organic adhesive layeris not required. The necessity for simultaneously forming holes in theinsulating base member and the adhesive layer by the laser process canbe eliminated. That is, after the holes have been formed in theinsulating base member by the laser process, the adhesive layer can beformed on the single-sided circuit board or the substrate having theconductive circuit. Therefore, the desmear process, which is performedafter the holes have been formed can be performed before the adhesivelayer is formed. As a result, the desmear process does not erode theorganic adhesive layer.

Also in a case where the non-penetrating hole is filled with theelectrolytic plating, the adhesive layer can be formed on thesingle-sided circuit board or the substrate having the conductivecircuit after the non-penetrating hole has been formed in the insulatingbase member by performing the laser process and the non-penetrating holehas been filled with the electrolytic plating. Therefore, theelectrolytic plating solution and the adhesive layer are not madecontact with each other. As a result, the erosion of the adhesive layerwith the plating solution can be prevented.

Since the adhesive layer is not hardened until the heat pressing processwhich is the final process is performed, the adhesive layer easilydeteriorates during the desmear process or the plating solution. Thepresent invention has characteristics that the above-mentioned problemcan be prevented and a reliable substrate can be easily formed.

Furthermore, in the single-sided board of the present invention, the viaholes are formed by enclosing the electrolytic plating in thenon-penetrating holes of the insulating base member. The projectingconductors made of the conductive paste or the metal having a lowmelting point are formed on the surfaces of the via holes formed on thesurface of said insulating base member opposite to the surface on whichconductive circuit has been formed. Therefore, since the conductivepaste or the metal having a low melting point is deformed during theheat pressing and absorption of dispersion of the heights of theelectrolytic plating can be realized, the multilayer printed circuitboard which the defective connection can be prevented and is highlyreliable in connection is obtained.

Moreover, a necessity for previously forming a conducting hole in theadhesive layer can be eliminated according to the present invention.Therefore, defective conduction caused from deviation in the positionsof the hole in the adhesive layer and the projecting conductors providedfor the organic insulating base member can be prevented.

In the present invention, the projecting conductors comprised theconductive paste or metal having a low melting point are formed on thevia holes filled with the electrolytic plating. Therefore, theelectrical connection between the upper and lower conductive layers canbe easily established by penetrating the relatively thin organicadhesive layer on the multilayer printed wiring board. Therefore, theheight and the diameter of each projecting conductor can be reduced.Hence it follows that the pitch between adjacent projecting conductorscan be shortened. Therefore, the pitch between adjacent via holes canalso be shortened. As a result, addressing to the raising of the densitycan be permitted.

1. A method of manufacturing a single-sided circuit substrate at leastcomprising the steps (1) to (4): (1) forming non-penetrating holes whichreach a metal layer by a laser process in a rigid insulating base memberhaving a single-side copper-clad laminates comprising of an aramidenonwoven fabric cloth-epoxy resin base member, a glass cloth epoxy resinmember, or an aramide nonwoven fabric cloth-polyimide base member havingcopper foil formed on either side thereof; or obtained by heat-pressinga laminate of prepreg and the copper foil, the prepreg being in a formof a B-stage constituted by causing a glass cloth to be impregnated withphenol resin or bismaleimide-triazine resin: (2) enclosing copperelectrolytic plating into said non-penetrating holes formed in step (1)to form via holes: (3) forming a conductive circuit by etching saidmetal layer; and (4) forming projecting conductors made of conductivepaste or metal having a low metal point on the surfaces of said viaholes formed on the surface of said insulating base member opposite tothe surface on which said conductive circuit has been formed so that asingle-sided circuit substrate is obtained.
 2. A method of manufacturinga single-sided circuit substrate at least comprising the steps (1) to(3): (1) forming non-penetrating holes which reach a metal layer by alaser process in a rigid insulating base member having a single-sidecopper-clad laminates comprising of an aramide nonwoven fabriccloth-epoxy resin base member, a glass cloth epoxy resin member, or anaramide nonwoven fabric cloth-polyimide base member having copper foilformed on either side thereof; or obtained by heat-pressing a laminateof prepreg and the copper foil, the prepreg being in a form of a B-stageconstituted by causing a glass cloth to be impregnated with phenol resinor bismaleimide-triazine resin; (2) enclosing copper electrolyticplating into said non-penetrating holes formed in step (1) to form viaholes and forming projecting conductors made of conductive paste ormetal having a low melting point on the surfaces of said via holesformed on the surface of said insulating base member opposite to thesurface on which said conductive circuit has been formed; and (3)etching said metal layer to form a conductive circuit so that asingle-sided circuit substrate is obtained.
 3. A method of manufacturinga single-sided circuit substrate according to claim 1, furthercomprising a step for coarsening a surface of said conductive circuit.4. A method of manufacturing a single-sided circuit substrate accordingto claim 2, further comprising a step for coarsening a surface of saidconductive circuit.